The problem which arises, especially in vessels with mixed mechanical/sail propulsion, of reducing the hydrodynamic resistance of the components forming part of the mechanical propulsion which project from the submerged part of the hull, such as shafts, supporting frames, propellers, turbines or any other appendages necessary for such propulsion (excluding those used in manoeuvering), when not in use, mainly when under sail, has been known for some time.
Up to now various solutions have been developed for achieving a maximum reduction in the hydrodynamic resistance of such propulsive components while under sail; among others there is fairing of the shafts and supporting frames, as well as the use of variable pitch propellers, using feathering or folding blades. As mentioned, expensive solutions of this type seek to achieve a maximum reduction in the major penalty imposed upon the performance of a vessel of the class mentioned by the existence of appendages which when not in use have no other function than to increase resistance to sailing. However, the results achieved with these leave much to be desired.
One of these known approaches comprises variable pitch propellers which are generally equipped with internal gears which are also capable of orientating the blades in the direction of the vessel's travel when the engine is stopped (feathering).
Another arrangement which is currently in use is that of propellers with folding blades which generally comprise at least two hinged blades connected together by gears which unfold after a specific number of revolutions and which fold up when the vessel is sailing without the engine in operation (through the thrust effect of the water acting upon them), in order to reduce the braking effect otherwise exerted by the said blades.
The result of the two arrangements considered is firstly that the appendages giving rise to the hydrodynamic resistance do not wholly disappear and secondly that all the propellers described have a poorer performance when in use than a propeller having a fixed pitch and diameter, with the result that in the case of vessels incorporating this type of arrangement the power of the engine has to be limited so as not to excessively increase the resistance caused by fitting a propeller of larger dimensions and, as a consequence, when sailing under motor propulsion this type of vessel cannot achieve the maximum speed which it could develop in view of its length.
Furthermore, and in the case of sailing vessels, the provision of auxiliary engine equipment provided with a fixed pitch propeller makes it necessary to install a braking system to prevent the shaft from rotating if it is desired that the mechanism should not be compelled to suffer vibration and wear if it rotates freely.
Likewise, continued exposure of the propulsion components to the marine environment gives rise to sticking and organic encrustation which affect their preservation and performance, especially during periods of prolonged inactivity.
Various documents comprising the state of the art are known in the patent literature, and in the applicant's opinion the most pertinent are the following:
U.S. Pat. No. 6,056,610 describes a transverse or longitudinal propulsion system associated with means to extend it from a well present in the hull of a vessel into an operating condition and retract it within the said well when it is not in use. Movement between these two extended and retracted positions respectively is brought about through an operating arm located along the geometrical axis of the well and along the continuation of this within the hull.
This type of propulsion system increases manufacturing costs and appreciably complicates transmission of the drive from the motor, unless this is located in the propulsion system itself, which further increases costs and limits the power available in relation to its size, for which reason its application is restricted to auxiliary manoeuvering systems, and never the main propulsion for the vessel.
U.S. Pat. No. 4,668,197 teaches a retractable auxiliary propulsion device designed for use on small vessels and comprising an engine/propulsion system assembly mounted above the waterline in inclined guides and housed in a compartment in the stern of the vessel when not in use. This device can be lowered into its operating position, sliding downwards along the said guides, so that its propulsion member, for example a propeller, is placed in the water. The assembly in question is provided with a shape which can also perform the function of a rudder through the operation of a hydraulic piston which orientates it in one direction or another. The upward and downward movement of this engine member is brought about through a hydraulic piston and a cable.
The subject matter of this document is an economical auxiliary propulsion device of low power and which can be used for vessels of small size only.
U.S. Pat. No. 4,678,440 describes a propulsion system for vessels which makes it possible to use these in shallow waters, in that the engine and the propeller shaft constitute a rigid assembly mounted in a tilting manner above the flat of the stern in such a way that the propeller can be submerged into the water to a greater or lesser extent, or completely removed from it. Control of the tilting of this engine, shaft and propeller assembly is brought about through a lever operated by a crew member and is incorporated with the said engine-propeller shaft assembly at a point close to the former, and this lever can incorporate controls for operation of the said engine.
This type of propulsion system can only be used for vessels of very small size, for example boats of the type used by fishermen or hunters who need to move in very shallow waters such as marshes, and nowhere in the document is the possibility of applying it to vessels of appreciable length and displacement mentioned.